In digital systems for mobile communication speech signals are encoded by a speech encoder in order to reduce the data rate and for saving bandwidth. In a call originating in a mobile terminal and terminating in a mobile terminal, a so-called mobile-to-mobile call, the speech signal usually is encoded and decoded twice. In the originating mobile terminal the speech signal is encoded a first time before the encoded signal is sent over the air to a first network node, e.g. a base station. A first transcoder decodes the encoded signal, which it receives from the first network node, into a so-called a-law/μ-law signal which is commonly used in fixed communication networks. The decoded signal is routed in the fixed network to a second network node, e.g. a second base station. Before the second network node can transmit the signal, the signal is encoded again in a second transcoder. The encoded signal is emitted by the second network node and is decoded in the terminating mobile terminal. The speech signal flow in the opposite direction is handled symmetrically.
As in this configuration two encoder/decoder pairs are lined up (the encoder of the originating mobile terminal and the decoder of the originating transcoder are regarded as a first pair and the encoder of the terminating transcoder and the decoder of the terminating mobile terminal are regarded as a second pair), this configuration is called a speech codec “tandem”. The key inconvenience of a tandem configuration is the speech quality degradation introduced by the double transcoding. This degradation is usually more noticeable when the speech codecs are operating at low transmission rates.
When the originating and the terminating mobile terminal are using the same type of speech codec, it is possible to transmit the speech frames received from the originating mobile terminal to the terminating mobile terminal without the need to activate the transcoding functions in the first and the second transcoder. As then there is only one pair of encoder and decoder (that is, the encoder in the originating mobile terminal and the decoder in the terminating mobile terminal) involved, this configuration is called Tandem Free Operation (TFO). In modern networks, like UTMS, it is even possible to discard the whole transcoder hardware. This is then called a Transcoder Free Operation (TrFO) instead of e.g. the usual a-law/μ-law signal. In TFO and TrFO mode the encoded and compressed speech signal is transmitted over the fixed network. Besides the improvement of the speech quality by avoiding double transcoding TFO also saves costs as the compressed signal needs less bandwidth in the fixed network and power is saved since transcoding is bypassed twice. All necessary methods for negotiating, establishing and maintaining a Tandem Free Operating connection (TFO connection) are standardized for codec types without configuration parameters (e.g. in GSM 08.62 for GSM_FR, GSM_HR and GSM_EFR) or for more complex codec types (e.g. the adaptive multi-code rate (AMR)) by the 3rd Generation Partnership Project (3GPP).
In Technical Specification 3G TS 28.062 V.5.0.0; 3rd Generation Partnership project; Technical Specification Group Services & Systems Aspects; In-band Tandem Free Operation (TFO) of Speech Codecs; Stage 3; Service Description; Release 5, which is exemplary in respect of bypassing serial transcoding operations, various aspects of TFO for 3GPP are illustrated. TFO is activated and controlled in 3GPP by so-called Transcoder Units after the completion of the call set-up phase at both ends of a mobile-to-mobile call configuration. The TFO protocol is fully handled and terminated in the Transcoder Units. For this reason, the Transcoder Units cannot be bypassed in TFO. This is the key difference in comparison with TrFO which is defined in 3GPP TS 23.153. In return, the Transcoder Units continuously monitor the normal TFO and can terminate TFO as soon as necessary with limited impact on the speech quality.
Before TFO is activated, the Transcoder Units exchange conventional 64 kbit/s PCM speech samples encoded according to a-law or μ-law. The Transcoder Units can also exchange TFO messages by stealing the least significant bit in every 16th speech sample (see annex A of 3GPP TS 28.062 V5.0.0 for the specification of the TFO message transmission rules and clauses 6 to 8 for the description of the TFO procedures and message contents).
If compatible speech codec types and configurations are used at both ends of the mobile-to-mobile call configuration, the Transcoder Units automatically activate TFO. If incompatible speech codec types and/or configurations are used at both ends, then a codec mismatch situation exists. TFO cannot be activated until the codec mismatch is resolved. This capability is an optional feature involving other network elements of the Radio Access Network (RAN).
Once TFO is activated, the Transcoder Units exchange TFO frames carrying compressed speech and in-band signaling, the structure of which is derived from the GSM TRAU frames defined in the 3GPP TS 48.060 and 48.061 (see clause 5). The is exchange of TFO messages is still possible while TFO is active. In this case, the stealing process will result in embedding a message in the synchronization pattern of the TFO Frame.
If the Transcoder Units find that the codec types currently used by both mobile terminals are compatible they will immediately enter into a TFO mode. Although the mobile terminals often support a codec type with better properties (e.g. better speech quality or better data rate) than the currently used codec type, very often they will not start with the optimal codec type because only the codec type currently in use is signaled. If on the other hand both radio subsystems would always report the best codec type supported by the respective mobile terminal no agreement will be found and the communication will be started in tandem mode, although a TFO mode would have been possible.
Although certain procedures ensure that in the course of the communication the communication connection can be switched to a common codec type and thus TFO can be enabled later on, the establishment is significantly delayed and the signal distortions in an initial tandem operation mode are inconvenient. Therefore, one could think about reporting a codec type with lower properties but that is widely spread in order to establish TFO very early. As further on messages are exchanged that report a list of supported codec types of each terminal, in the course of the connection the codec type in use may be changed to a better codec type. However the users of the mobile terminals will experience the change of the codec while the TFO is established as (clicking) noise and may be irritated.
It is an object of the invention to improve the bypassing of two or more transcoding operations that are performed in series. It is a further object of the invention to allow an efficient protocol version handling.